build(web): generate the UI's Tailwind CSS with the pinned standalone CLI (#685)

Replaces the hand-written Tailwind subset in app.css with a real, purged
Tailwind build: scripts/build_css.py runs the pinned, SHA-256-checked
standalone Tailwind CLI (no Node), the generated tailwind.css and
plugin-frame.css are committed, and CI fails when they are stale. The Pi
never builds anything. The login page (#683) now links tailwind.css too,
and the load-order test covers every template that links app.css.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/tailwind/plugin-frame.input.css

@@ -0,0 +1,12 @@
+/*
+ * Input for static/v3/plugin-frame.css, the stylesheet for plugin web_ui/
+ * fragments shown in an iframe. Don't edit the output; edit this file or
+ * plugin-frame.config.js and run `python3 scripts/build_css.py`.
+ *
+ * Includes Tailwind's preflight, as the v2 CDN build the fragments were
+ * written against did.
+ */
+
+@tailwind base;
+@tailwind components;
+@tailwind utilities;